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Erechin are described in Leviticus 27:1-8, and the idea behind them is discussed more here. The Torah assigns a simple "erech-value" to people based on their age and gender. Thus if someone pledged to donate "the erech of so-and-so", s/he would look up their age and gender in the erech table and be obligated to give that amount of silver.

A neder means a person vowed to bring a sacrifice. (See Rashi, Leviticus 22:18). So if a person said they hereby made a neder to bring, let's say, a shelamim sacrifice, s/he would be obligated to bring some appropriate animal as a sacrifice.